1. Field of the Invention
The present invention relates generally to electronic circuits, and more particularly to a ballast for an HID (High Intensity Discharging) lamp.
2. Description of the Related Art
A conventional HID lamp is a gas-discharging lamp, which must be activated by high voltage. When the HID lamp is activated to stay at steady state, it is devoid of current self-control and its illuminative property is subject to variation of electric voltage or frequency to further incur qualitative alteration or short longevity. Accordingly, an electronic ballast is required to eliminate the problems indicated above.
However, a conventional ballast is merely applicable to the general gas-discharging lamp. For a more high-voltage HID lamp used in, for example, a projector or a headlamp of the car, it is necessary to use a particular ballast of specific circuits that is made by any of the world-class manufactures. Hence, such particular ballast has high production cost and high price to incur commercial unpopularity.
A conventional electronic ballast 70, as shown in FIG. 7, for use in an HID lamp is powered with DC (Direct Current) battery power from a car, including a filtering circuit 71, a flyback conversion controller 73, a full-bridge converter 75, a high-voltage ignition circuit 77, and a control circuit 79. The DC passes through the filtering circuit 71 to be filtered, enters an input end of the flyback conversion controller 73 to be provided with the required power (15–310V) for high-voltage driving and keeping the HID lamp in transient state and steady state, then outputted from the flyback conversion controller 73 and inputted into the full-bridge converter 75 and an low-voltage end of the high-voltage ignition circuit 77; the output end of the full-bridge converter 75 is electrically connected in series to the HID lamp and the high-voltage ignition circuit 77, thereby defining a high-voltage series structure; and is converted into AC (alternate current) to enter the high-voltage ignition circuit 77 to generate high voltage to ionize the high-voltage gas inside the HID lamp, such that the HID lamp is powered to illuminate. The control circuit 79 is provided for manipulating the actions of the aforesaid elements so as to control the procedure of the illumination of the HID lamp.
However, the aforementioned conventional electronic ballast 70 still includes drawbacks as follows.
1. The output range (15–310V) of the flyback conversion controller is too large, incurring production difficulty and high production cost.
2. There are four switches in the full-bridge converter. The four switches each must have voltage reactance greater than 500V, being of high production cost, of high working temperature, and less reliable.
3. The high-voltage ignition circuit is in the series structure to have less turns of the secondary coil of the transformer T3, which coil diameter is large enough to cause little reactance, such that it is difficult for induction between the primary and secondary coils to incur worse stability and coherence of the production, further resulting in unstable quality. In addition, a switch 78 must have a spark gap of high price to process the voltage of higher than 2000V, but the spark gap is subject to damage to cause damage of the whole ballast 70 mostly.